Broadly the present invention relates to a new use for an old device comprising a new method of seismic exploration and improved explosive marine seismic energy pulse generation for the introduction of seismic energy into water in a conventional shot hole filled with water, marshy areas, or particularly offshore from a floating vessel for supplying the explosive material and having a geophysical crew thereon for exploring earth layers and formations underlying land or bodies of water as oceans, seas, lakes, rivers, or the like. More particularly this invention pertains to a few methods for generating and shaping a seismic energy pulse resulting in attenuation and time delay of secondary pulses, and for generating and shaping the seismic energy pulse.
While methods are applicable for many purposes as will be set forth further below, they are illustratively disclosed and described as applied to underwater sound instrumentation such as oceanographic equipment and systems that are employed in seismic exploration of the world's crust under and adjacent to bodies of water.
Since water is such a good sound conductor, it is unnecessary to generate sound waves right on or in the ocean floor; they can be produced in the water near the surface. The pressure or energy waves travel down through the water to the ocean floor and are reflected as in the usual echo-sounding techniques. However, these waves also penetrate into the ocean floor and are reflected from the sub-strata. These seismic acoustical waves also propagate horizontally through one or more geological strata and may be recorded at a distance from the source, thus providing useful refraction data on the stratum or strata involved.
Although conventional explosives for marine seismic work can put large amounts of energy into the water and obtain great depth of penetration, they do have drawbacks; they are dangerous to handle and use, and in some areas such as congested harbors, they cannot be used at all. Also, each "shot" is very expensive and can run into many thousands of dollars per survey. Explosives tend generally to concentrate substantial amounts of their energy output into higher frequency components which may not be desirable for many purposes; whereas, the sound impulse generation method for the present invention can be adjusted over a low amplitude range and adjusted in frequency so as to provide the desired spectrum distribution of sound frequencies for the purposes at hand.
The present invention is illustratively described as embodied in a device capable of emitting a large amount of acoustical energy into water in the form of a clear, repeatable pulse, the frequency and amplitude of which may be readily averred. These powerful sound impulses are well adapted for use in seismic exploration systems and also can be used to advantage for other purposes.
Seismic surveys which are conducted over water covered areas use various methods for generating seismic energy as by the detonating of conventional powder or dynamite, or gas mixtures, electrical discharge of sparks to ionize a portion of the water surrounding the electrode, or suddenly releasing a container of high pressure air underwater for generating a large seismic or pressure wave signal. These seismic signals are reflected from subsurface geological formations and structures and are received by seismometers and recorded.
In most methods utilized, it is customary to either position one or more detectors on the ground surface or tow them through the water in the vicinity of the sound source to detect the signals that are reflected from the various subsurface formations and structures. The detected signals are recorded on suitable equipment contained on the towing vessels, either in the form of analog or digital signals. Also, at times the signals are recorded as variable area signals to provide a profile of the surveyed area. The latter recording is similar to those contained with conventional depth-sounding equipment.
All of the above seismic methods and devices have various disadvantages and thus none is completely suitable for use in all circumstances. The handling of an explosive material is, of course, dangerous as mentioned previously wherein the explosions themselves tend to kill much marine life, an example being disclosed in U.S. Pat. No. 2,877,859. Also, in the case of dynamite two separate vessels are required; one for handling the dynamite and the other for the recording equipment. This, of course, increases the cost of the survey.
Aside from the operational advantages over conventional dynamite surveys, the disclosed system affords better data acquisition through the effective attenuation of the signal's secondary oscillations and by the effective use of the ghost, reflected, or rarefactional seismic energy pulses.
The use of explosive gas mixture solves some of the problems that arise with dynamite, since explosive gas mixtures do not kill as much marine life, a further example being disclosed in U.S. Pat. No. 3,620,327. Normally, gas mixtures can be stored as separate, non-explosive gases on the same vessel that contains the recording equipment. Thus, the need for an additional vessel or the possibility of killing marine life, is reduced. While one disadvantage of gas type sound sources is the relatively short life of the flexible container or sleeve in which the explosive gas mixtures are detonated, conservation of the present environment is one of the principal problems.
In sound sources employing an electrical discharge, it has been customary to discharge a bank of capacitors charged to a high voltage through a single electrode or multiple parallel electrodes and a ground plate. When the capacitor bank is discharged into the water by the electrodes and ground plate the pressure bubble is produced at each electrode tip, for the ground plate serves as a return for the electrical energy. The pressure bubble produces the desired seismic impulse while the ground adds nothing to the magnitude of the outgoing acoustical wave. In fact, only 3-5 percent of the total electrical energy available in the capacitor bank is converted to useful acoustical energy in the water. Further many spark electrodes in parallel may be used simultaneously as disclosed in U.S. Pat. No. 3,613,823.
Well known methods to attenuate the pulsation of energy bubbles consists in using a metallic sphere which is provided with a plurality of holes regularly distributed on its wall and in which the explosion is carried out, this sphere being made of a material which can withstand the explosion, as disclosed in U.S. Pat. Nos. 2,877,859 and 3,525,416. While, however, it is difficult to build such spheres having a sufficient mechanical strength to withstand numerous repeated explosions, complete attenuation of bubble pulse amplitude is still lacking. Another known device comprises a container with a resilient opening and closing element which provides an intermittent connection of the interior of the container with the surrounding liquid medium, as disclosed in U.S. Pat. No. 3,444,953. None of the above systems completely attenuate the amplitude of bubble pulsation or oscillations.
While the use of "Primacord" is old when combined with a main charge of dynamite, as disclosed in U.S. Pat. No. 2,599,245, for creating a gas escape path for elimination of the bubble and its accompanying ghost pulse, but no means is disclosed for shaping the bubble pulse by using the reflected energy, as for generating an elongated seismic energy pulse at the lower end of the explosives, particularly by utilizing the ghost pulse.
A feature that is known about "Primacords" is that the distance between the primary pulse and the ghost pulse varies relative to the depth of the "Primacord", from Geophysics, October 1953, No. 4, pages 793-804.
Further uses of "Primacord" as a powerful explosive are disclosed in U.S. Pat. No. 2,609,885 and U.S. Pat. No. 2,775,878 wherein the explosive is coiled or packed in tight helical coils to concentrate the effect of the "Primacord" to produce a greater pressure rise than TNT by "focusing". No use for generating vertical energy pulses with straight lengths of "Primacord" are suggested herein, nor is the primary pulse strengthened by the reflected energy for pulse shaping disclosed.
While the use of dynamite is very restricted in offshore exploratory operations for preservation of the ecology, the restrictions on the utilization of "Primacord," or the like, is much less, if any. Only the dynamite caps used to detonate the "Primacord" must be handled with caution, comparatively.
The U.S. Pat. No. 3,212,437, however, discloses an elongated line charge, the primary objects of which are to form an explosive device having sound waves which spread in predetermined planes, i.e., "predominantly horizontal plane, in which there are inconsequential bottom and surface reflections of the water." On the contrary or alternative, the disclosed invention pertains to a new use for the above known device of utilizing the above noted "inconsequential bottom and surface reflections" for shaping a seismic energy pulse to a precise shape as illustrated in FIGS. 2, 4, and 6.